In the design of radio transmission systems, one of the desires that have to be addressed is power efficiency. While the power efficiency was practically always important for battery-powered devices, it has also become an issue for the infrastructure of communication networks. Efforts to improve the power efficiency (among other aims) may be summarized under the keyword “Green IT”. Green IT not only pertains to computers and servers but also to associated subsystems, such as monitors, printers, storage devices and networking and communications systems. Besides the need to comply with Green IT standards, network operators may expect additional advantages from employing power efficient devices and systems, such as reduced energy costs.
In the field of mobile communications systems another development can be observed. Modern communication standards sometimes require linear power amplification in contrast to, for example, frequency-modulation (FM) based systems for which highly non-linear transmitters and receivers were sufficient. Linear power amplification compromises overall system energy efficiency because radio frequency (RF) power amplifiers (PAs) are less efficient when they are operated at a power level that is below the power amplifier's maximum power (backed-off operating mode). It has been estimated that the power amplifier and its associated components consume up to 50% of the overall power in a cellular base station.
Modern mobile communications standards such as UMTS, WiMAX and Long Term Evolution (LTE) are intended to support a variety of different channel coding and modulation techniques (CDMA, OFDM, etc.), broader channel bandwidths and high peak-to-average power ratio (PAPR) modulation schemes. Moreover, there is an increasing demand for base stations supporting a wider range of frequency bands. Such base stations are more flexible and may be reconfigured if new frequency slots become available or a frequency slot gets assigned to a new communications standard, such as UMTS.
These developments place a high demand on the power amplifiers in the base transceiver station, in particular, with respect to linearity, bandwidth and power efficiency. Linearity and power efficiency tend to contradict each other. In the past, several solutions were proposed to overcome this dilemma.
One of these solutions is envelope tracking. Envelope tracking is being widely researched and is undergoing development in a number of companies. Envelope tracking means that the supply voltage of the radio frequency amplifier is adapted to the instantaneously needed output power. For low output powers the supply voltage is reduced and for high output powers it is increased. The power amplifier may be driven near its (instantaneous) maximum output power where it has maximum efficiency if the supply voltage follows the instantaneous output power in a continuous manner. It is also possible to adjust the supply voltage in a step-wise manner which usually still results in good power efficiency for the power amplifier.
The LTE standard has a relatively high peak-to-average ratio. Envelope tracking usually offers good power efficiency for signals having a high peak-to-average ratios. Therefore, envelope tracking appears to offer a compelling efficiency case for LTE signals and similar standards.
U.S. Patent Application Publication No. US 2009/0045872 A1 discloses an integrated transceiver with envelope tracking. A power supply is operable for varying a level of power supplied to an amplifying circuit (by varying the voltage of the supply) in response to the variation of an input signal envelope directed to the power supply. The processing circuit is operable for digitally detecting the input signal envelope to provide variation of the power supply level. The processing circuit includes an envelope pre-distortion circuit operable for receiving and pre-distorting the input signal envelope before it is directed to the power supply. The entire disclosure of U.S. Patent Application Publication No. US 2009/0045872 A1 is incorporated herein by reference.
Envelope tracking is also discussed in UK Patent Application GB 2457764 A, in US Patent Application Publication Nos. US 2009/0128236 A1 and US 2007/0210771 A1, and in the International Application published under the International Publication No. WO 2007/113726 A1. The entire disclosures of the foregoing applications are incorporated herein by reference. The mentioned documents disclose transmitters or transceivers having a single transmission path. The envelope tracking structure is exclusively dedicated to the single transmission path.